I'll submit a couple of pull requests later this week.
As I said, I'd rather avoid the conditional compilation when possible. Some of the changes are removing functionality that doesn't make sense -- like running Windows installers, DOS-style batch files, Powershell scripts, etc, on Linux. Others are using alternate functionality -- for example, the Scintilla library doesn't work on Linux. There are also places where Win32 API code is called for an underlying object, even though there's a .NET method. I'm not sure if you're working around a bug of some sort or what. One example of this can be found in ControlRedrawLock. .NET provides SuspendLayout and ResumeLayout methods which work fine on Linux under Mono, but calling SendMessage obviously would not. There's enough of this, especially the disabling of power shell, running the installers, etc., that I think conditional compilation does start to get messy. I'm not looking forward to maintaining it, anyway.
The only one I'm aware of so far is the use of MyDownloader, which is under the Code Project Open License (CPOL). See https://www.gnu.org/licenses/license-list.html#cpol for info on why it's incompatible with the GPL. I've asked the original author if he would consider releasing it also under the GPL or another license which is compatible, but I haven't seen a response yet. Fortunately, you appear to be using only its most basic functionality, which is simple enough to replicate, although still annoying. Some licenses, like MIT and BSD-derived licenses don't require that you re-license the code in order to distribute it, so either of those would be compatible with CPOL code. The GPL requires that everything on which the code depends either be GPL or in a separate library that came with the operating environment. As I said, it's a grey area for original authors, and it's accepted practice to distribute an original work under the GPL with some dependent code that's not under the GPL, but it's always been clear that derivative works can't do that.